1. Field of the Invention
This invention relates to aircraft turbofan engine fan duct reversers and cowls and particularly to bifurcated fan ducts where the inner and outer duct walls are constructed as a single assembly and tied together by radially extending sidewalls to form a cowl.
2. Description of Related Art
Aircraft turbofan engines typically include a fan bypass duct where the majority of the air pressurized by the fan is passed though and which typically includes a fan nozzle for producing thrust from the fan pressurized bypass air. The remaining air is passed through the core engine in which it is used as the working fluid to help generate power for the fan. Surrounding the fan duct is an outer cowl and surrounding the core engine cavity is an inner cowling that is split into two portions, forward and aft, wherein the aft portion may be referred to as a core engine cowl that extends aft till about the end of the turbine section of the engine. Between the radially inner and outer cowls is the fan duct including at its exit a fan nozzle.
An aerodynamically clean fan duct, one that is smooth, designed to prevent flow separation, and aerodynamically shaped for minimum pressure and flow losses, is important to the efficient operation of the aircraft. Typical fan ducts are constructed of aluminum sheet metal that is light weight and suitable for forming the aerodynamically contoured shape of the fan duct and its walls, particularly of its nozzle. The sheet metal fabrication is typically accomplished using rivets and is often attached to a minimal frame structure for rigidity.
Turbofan engines often have thrust reverser apparatus mounted within the outer cowl as described in U.S. Pat. No. 3,541,794 entitled "Bifurcated Fan Duct Thrust Reverser", by Johnston et. al., issued Nov. 24, 1970, assigned to the same assignee, and incorporated herein by reference. Johnston notes that the need to have thrust reversers rigidly secured to the fan casing surrounding the fan duct makes access to the core engine difficult. To that end Johnston discloses a bifurcated fan duct having two semi-circular fan ducts that are substantially mirror images of each other. Each semi-circular duct has radially spaced apart generally arcuate inner and outer walls connected by rivets, welding, or the like to radially extending sidewalls at the circumferential ends of the inner and outer walls.
The core engine cowling is provided by two axially extending sections, the first an inner wall of the fan ducts and the second or aft section an aft or core engine cowl. A fan cowling is provided by two axially extending sections where the aft section is the outer wall of the bifurcated fan duct. The outer wall is a fan reverser cowl containing fan thrust reverser apparatus including actuating mechanisms, turning vane cascades, and stowed blocker doors.
Previous fan duct assemblies have been fabricated using rivets for attaching sheet metal forming the wetted aerodynamic surfaces made of the duct. This type of construction has several inherent drawbacks. The design is inherently heavy and the aluminum used in the fabricated assembly requires insulation blankets and thermal barriers to protect the duct from the heat of the engine and for fire protection. This further adds to the weight of the engine and makes access to the core engine more difficult.
The rivet heads protruding from the surface of the fan duct walls into the fan bypass airflow causes flow and pressure losses which reduce engine fuel efficiency. A fabricated duct, one made of an assembly of parts, is less rigid and weighs more than a single continuous duct constructed of a composite material such as the present invention.
The term composite, for the purposes of this invention, denotes a material containing a fiber such as a carbonaceous, silica, metal, metal oxide, or ceramic fiber embedded in a resin material such as Epoxy, PMR-15, BMI, PEEU, etc. of particular use are fibers woven into cloth that is impregnated with a resin and cured via an autoclaving process or press molding to form a hard relatively homogeneous article. The composite suggested in the preferred embodiment is a graphite woven cloth impregnated with a PMR-15 polyimide resin, this includes tape as well as fabric. A discussion of this material may be found in an article in the January 1990 issues of Plastics Engineering, entitled "Processing of PMR-15 Prepregs for High Temperature Composites", by Mel Kaniz, which is incorporated herein by reference. Further information concerning suitable materials and definitions may be found in the "Engineering Material& Handbook" by ASM INTERNATIONAL, 1987-1989 or later editions, which is incorporated herein by reference.